Improvements to solar cell efficiency and radiation hardness that are compatible with low cost, high volume manufacturing processes are critical for power generation applications in future long-term NASA and DOD space missions. We consider the physics based models, and simulations of the radiation effects in a novel, ultra-thin (UT), Si photovoltaic (PV) solar cell technology, Figure 1. Such solar cells have a potential to achieve high conversion efficiencies while shown to be lightweight, flexible, and low-cost, due to the use of Si high volume manufacturing techniques. To achieve high efficiency on thin wafers Regher Solar is using amorphous/crystalline silicon heterojunction technology and a novel contactless metallization technology based on electroplating which can enable ultrathin silicon solar cells with up to 23% AM0 efficiency. Flexible light-weight solar panels made of UT Si solar cells can reduce solar array mass, volume, and cost for space missions. When solar cells are used in outer space or in Lunar or Martian environments, they are subject to bombardment by high-energy particles, which induce a degradation referred to as radiation damage. Radiation tolerance (or hardness) of this UT Si PV technology is not well understood. Research, review, and analysis of solar-cell radiation-effects models in literature have been conducted, and physics-based models have been selected and validated . Several different engineering approaches have been investigated to improve Si solar cell radiation hardness. Other approaches include Material/ Impurity/Defect Engineering (MIDE), Device Structure Engineering (DSE), and device operational mode engineering (DOME), which have been shown to be effective in reducing the effects of displacement damage in Si based devices . Lithium-doped, radiation-resistance silicon solar cell is considered an attractive experimentally proven possibility as well . In this paper, we provide the results of numerical simulation of the radiation effects in UT Si PV cells, and review radiation damage mitigation techniques. The results of numerical simulation of the radiation effects, coupled with the phenomenon of non-uniform vacancy creation (i.e., maximum displacement damage occurs near the Bragg peak, as described earlier), further indicate that a high-energy protons will cause minimal damage in the ultra-thin 50μm (or thinner) Si solar cell. These results show that the UT Si PV cell technology can be used for space applications in the high radiation environment.